Twenty-four chromosome FISH in human IVF embryos reveals patterns of post-zygotic chromosome segregation and nuclear organisation.

Fluorescence in situ hybridisation (FISH) was first applied on in vitro fertilisation (IVF) embryos for the preimplantation genetic diagnosis of sex, then chromosome translocations and later for chromosome copy number (PGS). Because of the controversy surrounding PGS diagnostically, it has been replaced by array-based approaches; however, FISH remains a powerful tool for investigating mechanisms of both post-zygotic segregation error and nuclear organisation, especially if most or all of the chromosomes in the karyotype can be analysed. The purpose of this study was to develop and apply a 24 chromosome FISH assay to investigate chromosome-specific rates of gain and loss, nuclear organisation patterns and the veracity of the original PGS result in days 5-6 human embryos. Analysis of 17 embryos by this newly developed approach gave strong signals for all chromosomes; it revealed chromosome copy number for each human chromosome per cell for each embryo and the nuclear address of the (mostly centromeric) loci probed. As all embryos were surplus to IVF requirements for both transfer and freezing (and many had an abnormal PGS indication) expected high levels of chromosome abnormalities were seen and no single nucleus displayed a normal complement; all were mosaic. Certain patterns emerged, however, namely that chromosome loss was more common than gain and apparent mitotic non-disjunction. Moreover, the centromeric probes tended preferentially to occupy the nuclear centre. Where we had a prior day 3 biopsy PGS result, it was confirmed, in part, by 24 colour FISH in most but not all cases.

Is there a paternal age effect for aneuploidy?

Finding a positive association between paternal age and the incidence of aneuploidy is not difficult. A cursory analysis however reveals that any association is indirect, brought about by a close correlation between paternal age and maternal age. Approaches for dissecting out the confounding age effects of the mother has led to a lively exchange among epidemiologists, with perhaps a consensus for the absence of a paternal age effect, at least for trisomy 21. Molecular studies revealed the relatively minor contribution of paternal errors to trisomy, but even research on the paternally derived trisomies alone has been inconclusive; thus studies focussed directly on the sperm heads. Human-hamster fusion assays were superseded by FISH for establishing any possible link between age and the proportion of disomic sperm in an ejaculate. Despite innumerable microscope hours however, although convincing studies suggesting an age effect for disomies 1, 9, 18 and 21 and the sex chromosomes are in the literature, others failed to notice any association for these or other chromosomes. It is biologically plausible that chromosomal non-disjunction errors should increase with age. Male reproductive hormone production, testicular morphology and semen parameters all decline slowly with age and paternal age is implicated in congenital birth defects, such as achondroplasia and Apert syndromes and also linked to compromised DNA repair mechanisms. Despite several decades of epidemiological and molecular cytogenetic studies, however, we are still not close to a definitive answer of whether or not there is a paternal age effect for aneuploidy. In this review we conclude by questioning the efficacy of FISH because of difficulties in detecting nullisomy and because of evidence that the centromeres (from which most sperm-FISH probes are derived) cluster at the nuclear centre. Array-based approaches may well supersede FISH in addressing the question of a paternal age effect; for now, however, the jury is still out.

Nuclear organization in human sperm: preliminary evidence for altered sex chromosome centromere position in infertile males.

BACKGROUND: Many genetic defects with a chromosomal basis affect male reproduction via a range of different mechanisms. Chromosome position is a well-known marker of nuclear organization, and alterations in standard patterns can lead to disease phenotypes such as cancer, laminopathies and epilepsy. It has been demonstrated that normal mammalian sperm adopt a pattern with the centromeres aligning towards the nuclear centre. The purpose of this study was to test the hypothesis that altered chromosome position in the sperm head is associated with male infertility. METHODS: The average nuclear positions of fluorescence in-situ hybridization signals for three centromeric probes (for chromosomes X, Y and 18) were compared in normoozoospermic men and in men with compromised semen parameters. RESULTS: In controls, the centromeres of chromosomes X, Y and 18 all occupied a central nuclear location. In infertile men the sex chromosomes appeared more likely to be distributed in a pattern not distinguishable from a random model. CONCLUSIONS: Our findings cast doubt on the reliability of centromeric probes for aneuploidy screening. The analysis of chromosome position in sperm heads should be further investigated for the screening of infertile men.